Energy generation from waste heat using the carbon carrier thermodynamic cycle

ABSTRACT

The invention relates to a method is provided which allows the generation of high temperatures, e.g. above 120° C. and maximum 200° C. and low temperatures, e.g. below minimum minus 20° C., from waste heat or geothermal heat or similar heat sources having a temperature of between 20 and 70° C. The method may use essentially pure carbon dioxide as primary working fluid in an essentially closed loop as described in previous disclosures, alternatively low boiling solvents are employed. The common feature of different embodiments is that the system operates at least partly, specifically in the absorber or cold side of the process, below atmospheric pressure (1 bar). In the method a heat pump or heat transformer is realized within the technical boundaries mentioned above. The invention also relates to the use of the method in combination with a district heating system for elevating the temperature of the district heating medium or for electricity production on demand.

FIELD OF THE INVENTION

The present invention relates to a method for generation of energy wherea heat source supplies thermal energy in the range 30-80° C. to aRankine cycle including a desorber on the hot side, an absorber orliquefier on the cold side, heat exchangers and at least one compressorand at least one pump for working fluid, and at least one of thefollowing product streams is obtained:

-   -   thermal energy in the range +90, +100, +120 and maximum +200°        C.,    -   thermal energy in the range +15, +10, +5, 0, −5 and minimum −20°        C.,    -   electricity from the decompression of CO₂ or a working fluid        following the extraction of heat.

BACKGROUND OF THE INVENTION

Reference is made to PCT/SE/2012/050319 or WO 2012/128715 with priorityfrom SE 1100208-6 (filed Mar. 22, 2011), U.S. application 61/468 474(filed Mar. 28, 2011) and SE 1100596-4 (filed 16.08.2011), thesedocuments are included by way of reference. These documents describe anovel thermodynamic cycle for energy and cold production from heat below160° C., called C3 or “Carbon Carrier Cycle”.

Reference is also made to SE 1200711-8 and SE 1200554-2 (earliestpriority Sep. 11, 2012), now submitted as PCT application. Reference isalso made to recent disclosures SE 1400027-7, SE 1400186-1 (turbines),and SE 1400160-6. These disclosures, also included by way of reference,describe various preferred embodiments and improvements of the C3 cycle.

This invention relates to the field of heat pumps. Essentially, a heatpump uses one part electrical energy to generate more than one, often3-5 parts thermal energy, e.g. for warming of houses. The ratio of heatenergy generated to electrical input energy is called COP or coefficientof performance. Many embodiments are known, e.g. heat pumps which useground heat of around 10° C. to heat a cold gas which later iscompressed and transfers heat to a household water heating system (oftenbetween 50-90° C.). The gas, typically being propane, butane or R134a orsimilar HFC (hydrogen-containing fluorocarbon), is expanded therebycooling down, for the cycle to start again. Some heat pumps working withCO₂ gas only are also known.

Examples of prior art are also WO 2006/124776 (DuPont de Nemours),disclosing a heat pump using at least one refrigerant and at least oneionic liquid, preferably both fluorinated, and WO 2004/104399 (Dresser),which discloses the coupling of compressor and expansion device on onerotating shaft as a means of increasing the efficiency of a heat pump orair conditioning machine, see e.g. FIG. 4 in said disclosure.

The improvement needs for heat pumps are evident from the following: a)Lower paraffins as working fluids are flammable and pose thereforerisks. b) The often used HFC are expensive, and whilst they are notdangerous for the earth's ozone layer, they contribute to the greenhouseeffect. Emissions to the environment have to be avoided, causing strictregulations and costs for disposal and repair. c) Heat pumps accordingto the state-of-the-art operate at relatively high pressure (e.g. above30 bar), causing high equipment costs. d) For profitable operation ofheat pumps, it would be desirable to increase the COP to clearly above5.

This text discloses various solutions of said problems and describesdifferent embodiments. The common feature of the different embodimentsof the invention is that a low pressure system of high efficiency hasbeen realized whereby the pressures and temperatures in the process arein the following specified ranges:

-   Desorber: 0.5-5 bar, 30-80° C.-   Heat exchanger for heat production: 5-50 bar, 100-250° C.,-   Absorber: 0.01-0.9 bar, 10-50° C.

Specifically the absorber or cold side of the process operates alwaysbelow atmospheric pressure. It is unexpected and surprising that asystem according to the invention can operate reliably as prior artsuggests that air ingress and thereby performance deterioration isunavoidable. The system is therefore operated together with a separatedevice for concentration and ejection of non-condensable gases, see alsoSE 1400182-0 and 5E1400349-5 (submitted 04 April and 08 July 2014resp.).

SUMMARY OF THE INVENTION

The object of the invention is thus achieved by a method for generationof energy where a heat source supplies thermal energy in the range30-80° C. to a Rankine cycle including a desorber on the hot side, anabsorber or liquefier on the cold side, heat exchangers and at least onecompressor and at least one pump for working fluid, and at least one ofthe following product streams is obtained:

-   -   thermal energy in the range +90, +100, +120 and maximum +200°        C.,    -   thermal energy in the range +15, +10, +5, 0, −5 and minimum −20°        C.,    -   electricity from the decompression of CO₂ or a working fluid        following the extraction of heat,        characterized by that the pressure in the absorber or condenser        section are always below 1 bar and that the pressures and        temperatures in the process are in the following specified        ranges:

-   Desorber: 0.5-5 bar, 30-80° C.

-   Heat exchanger for heat production: 5-50 bar, 100-250° C.,

-   Absorber: 0.01-0.9 bar, 10-50° C.

Preferred embodiments are defined in the appending dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below in the form ofnon-limited examples, reference being made to the appended drawings, inwhich

FIG. 1 shows the heat pump according to the invention in one specificembodiment whereby the working fluid comprises CO₂ and amines, and

FIG. 2 shows a different embodiment whereby the working fluid compriseslow boiling solvents with boiling points at atmospheric pressure of50-100° C.

DESCRIPTION OF PREFERRED EMBODIMENTS

The heat pump is shown in one embodiment in FIG. 1.

The heat pump according to the invention operates as follows: heat, e.g.geothermal or waste heat of e.g. 40-70° C., is supplied via heatexchanger (1) (flow in at (2), flow out at (3) to a mixture of CO₂ andabsorbent (CO₂-rich absorbent mix). This mixture, as described in thea.m. disclosures and essentially comprising amines (including ammonia,NH₃) and CO₂, e.g. in the form of carbonates or carbamates, is pumpedusing pump (13) into the heat exchanger through pipe (14). In thedesorber (1), CO₂ gas is desorbed at a pressure dependent on the natureof the absorbent, but typically being 0.3 to 2 bar. CO₂ gas iscompressed in compressor (4) to a higher pressure, e.g. 5-20 bar, andthe CO₂ is heated by compression. This heat is extracted in heatexchanger (5), and the heat is transferred to a medium through pipes (6)and (7). The colder gas is expanded through an expansion machine (8)which preferably is mechanically coupled to (4) to save energy.Compression requires electrical energy, decompression delivers energysuch that it is desired to minimize the energy requirement. Depending onthe configuration, the process from stage (4) to stage (8) (compressionand decompression) may deliver (electrical) energy provided the pressureat (1) is higher than at (9). During the decompression at (8) and (9),the CO₂ gas cools down to very low temperature, e.g. minus 50° C. Thecold is extracted in heat exchanger (9) using medium flowing through(10) and (11). The gas is further led into absorber (12) where it iscombined with CO₂-lean absorbent mix transported through pipe (17) fromthe desorber (1). An absorption device as described in the a.m.disclosures is used preferably, e.g. a spraying device optionallycombined with multiple pass, counter-current absorption, wiped filmtechnology, cooling during absorption etc. It is useful to employ pump(18) to generate the necessary pressure (e.g. 3 bar) to accomplish theideal spray characteristics, i.e. small droplets of absorbent whichreact quickly with CO₂ gas from (9). It can be useful to couple pumps(13) and (18) in order to save energy.

Heat is generated during the chemical reaction of absorbent and CO₂.This heat may be removed using a heat exchanger in combination with theabsorber (12). This heat may be discarded or it may be used forpre-heating the CO₂-rich absorbent mix prior to entry into desorber (1),this option is not shown in the figure. The pre-heating is preferablyachieved by using a heat exchanger (19) in which hot, lean absorbent inpipe (17) transfers its heat energy to incoming rich absorbent in pipe(14).

The method allows the construction of small- or large-scale heat pumps.Useful heat source inputs are geothermal sources of 40-70° C., wasteheat from power plants, waste heat from the conventional operation of C3cycle as disclosed previously, and many other heat sources. Variousembodiments are conceivable where the operation is adjusted to the wasteheat sources available. The chemistries disclosed in a.m. applicationsdescribe a range of systems which are useful for various configurations.The high COP can be explained by the fact that, differing fromconventional heat pumps, compression energy can partly be recovered inthe decompression device.

Transport of liquids through heat exchangers etc only requiresrelatively low amounts of energy.

In summary, a novel heat pump technology is disclosed allowing the useof essentially pure CO₂ as working fluid with benefits forsafety-of-operation, simultaneous generation of heat and cold, possiblyco-generation of electricity or requiring limited input of electricalenergy, thus providing a very high COP, exceeding, depending on theconfiguration, a COP of 3-5.

Example: A heat source supplies 1000 kW thermal energy, the heatingmedium is cooled from 70 to about 40° C. 1 kg CO₂ per second isliberated in the desorber from an absorbent system comprising amines asdislosed previously. Compression of 1 kg CO₂/s from 2 to about 20 barrequires 100 kW energy and generates roughly 100 kW thermal energy, e.g.in the form of a steam flow at 150° C. through outlet 6. This heat maybe used in a paper mill for drying purposes or any other need includingpower generation. Cooling of CO₂ leads to a pressure decrease. CO₂ isfurther decompressed using e.g. a turbine generating (order ofmagnitude) 100 kW and is cooled. Cold is extracted, about 30 kW of cold(at −15 or −20° C.) can be extracted at heat exchanger 9. During theabsorption of CO₂ by amine in 12, an equilibrium temperature of about25-40° C. is reached, this heat may be partly removed by heatexchanging. Roughly 900 kW (thermal) are removed from the system bycooling, through cooling of the absorber 12 or cooling of lean, hotamine prior to transport into the absorber. With typically requiredpumping energies for absorbent in the order of 10-20 kW, a COP of(produced thermal energy) divided by (electrical power input)=130kW/20=6.5 or higher is achieved.

Referring now to FIG. 2, a different embodiment is disclosed. A workingfluid such as a ketone (acetone, MEK), an alcohol (methanol, ethanol,isopropanol), a paraffin (such as pentane), ammonia or amines, alone orin combination with water or water alone is pumped by pump (13) to thehot side of the process where at least part of the working fluid isevaporated in desorber (1), powered by thermal energy input (2) and (3).Some working fluid may be recycled for practical reasons (not shown).The working fluid enters compressor (4), is heat exchanged (5) andenters turbine (8) whereupon working fluid condenses in absorber (9).Condensed working fluid may be recycled to absorber (9) using pump (18)and cooler (16), however, main pump (13) may be used for that purpose ina modified scheme. Preferably, working fluid is sprayed into absorber(9) for efficiency and reduction of volume. A suitable system forconcentration and ejection of non-condensable gas such as air isdescribed in a separate disclosure (SE 100349-5) (see above).

In order to extract cold from expanding working fluid, it is preferredto use working fluids such as CO₂ and/or ammonia. Substantial heatextraction at (6) can pre-cool the working fluid. Condensable parts ofthe working fluid may be collected prior to turbine entry (8). Thesefeatures which are obvious to persons skilled in the art are omitted inthe drawings.

The common feature of embodiments 1 and 2 is that both systems operatepartly below atmospheric pressure.

Both embodiments as in FIGS. 1 and 2 are useful in combination withdistrict heating systems. Once the hot stream supplying heat to houseshas cooled down from e.g. 80° C. to 50 or 40° C., this stream can beheated using the heat pump according to the invention. The advantagesare that smaller pipes may be used for supplying district heat, as thesame flow is used to supply more heat, in addition the return flow tothe heat production unit can be lower in temperature, compared tooperation without the heat pump. This may increase the efficiency ofheat generation or in some cases simultaneous electricity generation.

In the figures the reference characters used denote the following.

FIG. 1:

-   1 Heat exchanger, waste heat input, rich absorbent mix desorbs CO₂;-   2 and 3 inflow heat, outflow heat source;-   4 compressor, compresses and heats CO₂ gas desorbed at 1;-   5 heat exchanger for extraction of high temperature;-   6 and 7 heat exchanger medium, e.g. liquid or steam;-   8 decompression, e.g. expansion machine, preferably coupled to 4,    e.g. on same axis;-   9 (and 10/11) heat exchanger for extraction of low temperatures    after expansion of gas;-   12 absorber, e.g. spray chamber with coupled heat exchanger (through    15/16);-   13 pump, for transport of CO₂ loaded absorbent mix to 1, 14 pipe to    desorber heat exchanger 1;-   17 pipe for transport of CO₂-lean absorbent mix to absorber 12;-   18 pump for transport of lean absorbent to absorber, preferably    coupled to 13;-   19 heat exchanger, for transfer of heat from lean to rich absorbent.

FIG. 2:

-   1 Heat exchanger, waste heat input, working fluid evaporation;-   2 and 3 inflow heat, outflow heat source;-   4 compressor, compresses and heats gaseous working fluid desorbed at    1;-   5 heat exchanger for extraction of high temperature;-   6 and 7 heat exchanger medium, e.g. liquid or steam;-   8 decompression, e.g. expansion machine, preferably coupled to 4,    e.g. on same axis;-   9 (and 10/11) heat exchanger for extraction of low temperatures    after expansion of gas;-   12 supply line for condensed liquid from condenser (9) to pump (13);-   13 pump, for transport of working fluid to 1;-   14 pipe to desorber heat exchanger 1;-   18 pump for transport or recycling of working fluid to absorber,    optionally coupled to 13;

1. A method for generation of energy where a heat source suppliesthermal energy in the range 30-80° C. to a Rankine cycle including adesorber on the hot side, an absorber or liquefier on the cold side,heat exchangers and at least one compressor and at least one pump forworking fluid, and at least one of the following product streams isobtained: thermal energy in the range +90, +100, +120 and maximum +200°C., thermal energy in the range +15, +10, +5, 0, −5 and minimum −20° C.,electricity from the decompression of CO₂ or a working fluid followingthe extraction of heat, wherein the pressure in the absorber orcondenser section are always below 1 bar and that the pressures andtemperatures in the process are in the following specified ranges:Desorber: 0.5-5 bar, 30-80° C. Heat exchanger for heat production: 5-50bar, 100-250° C., Absorber: 0.01-0.9 bar, 10-50° C.
 2. The methodaccording to claim 1 for co-generation of heat and cold and optionallyelectrical energy, using a heat source such as geothermal heat or wasteheat in the range 30-80° C., comprising the following steps in anessentially closed loop: a) condensing and cooling a working fluid suchas acetone, ethanol, methanol, isopropanol, ammonia, or water, alone orin any stoichiometric combination, on the cold side of the process(absorber), b) pumping said working fluid to the hot side of theprocess, c) evaporating said working fluid using said geothermal orother waste heat source and compressing said working fluid from 0.5-5bar to above 5 bar, or above 10 bar, preferably higher up to 50 bar, d)extracting heat from said compressed working fluid and sending said heatto a point-of-use, e) decompressing said working fluid whereupon cold isoptionally extracted and sent to a different point-of-use, f) condensingsaid working fluid (step a) for the cycle to start again.
 3. The methodaccording to claim 1 for co-generation of heat and cold and optionallyelectrical energy, using a heat source such as geothermal heat or wasteheat in the range 30-80° C., comprising the following steps in anessentially closed loop system: a) temporarily absorbing CO₂ as workingfluid in a suitable absorbent comprising an alkaline medium such as atleast one amine, b) desorbing CO₂ from the absorbent using saidgeothermal or other waste heat source and compressing CO₂ from 0.3-3 barto above 5 bar, or above 10 bar, preferably higher, c) extracting heatfrom said compressed CO₂ and sending heat to a point-of-use, d)decompressing CO₂ whereupon cold is optionally extracted and sent to adifferent point-of-use, e) re-absorbing CO₂ in said absorbent system,for the cycle to start again.
 4. The method according to claim 1,whereby a separate device is concentrating and ejecting non-condensablegas such as air from the process which at least partly operates belowatmospheric pressure.
 5. The method according to claim 1, where moreelectricity or work is consumed in the compression stage than isgenerated during the decompression.
 6. The method according to claim 1,where the energy consumption of the method is reduced by coupling thegas compression for heat generation with the gas decompression, bymechanical, electrical or other means.
 7. The method according to claim1, in which a heat pump or heat transformer is realized within thetechnical boundaries of above mentioned claims.
 8. Use of the methodaccording to claim 1, in combination with a district heating system forelevating the temperature of the district heating medium or forelectricity production on demand.